[sim] Add NetworkTables view to Simulation GUI (#2327)

This enables seeing the tables locally, including update timestamps and
external connections.

simulation/halsim_gui/src/main/native/cpp/NetworkTablesGui.h

@@ -0,0 +1,17 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+namespace halsimgui {
+
+class NetworkTablesGui {
+ public:
+  static void Initialize();
+};
+
+}  // namespace halsimgui